Analog-to-digital converters are used in many applications. Known analog-to-digital converters, however, have been limited by their sampling rate. Presently, the fastest known analog-to-digital converters have sampling rates below 10Gigasamples per second (GsPs). Most analog-to-digital converters have a sampling rate below one GsPs.
Another limitation of presently known converters is that aperture jitter is typically about one psec. Aperture jitter is the amount by which the sampling window may vary. Aperture jitter lowers the sampling rate capability of the analog-to-digital converter.
Optoelectronic analog-to-digital converters are known that use the electrooptic effect in Mach-Zehnder interferometers. In these previous optoelectronic A/D converters, a plurality of waveguide Mach-Zehnder interferometers are arranged in parallel. An optical pulse input is divided in power and fed to all of the interferometers. The electrical analog input is also divided in power and fed to all of the interferometers. Each interferometer contains an electrode of different lengths, with the lengths chosen to produce a high-light "1" or a low-light "0" output from that interferometer according to the level of the analog input. The previous analog-to-digital converters have been realized using titanium-diffused lithium-niobate waveguide structures. The resolution of such converters has been limited by the small electrooptic coefficient of lithium-niobate. In such devices, the length for interaction between the analog input signal and the optical pulse is very long. Because of this, the device has a length of six centimeters. This is a very long package with respect to microelectronic applications. Another characteristic of such devices is that very high voltages must be used to power the devices. One known device, for example, uses a peak-to-peak voltage of 16.8 volts. The device has only a four bit accuracy. When based on Mach-Zehnder interferometers, the smallest voltage increment that may be sensed by the A/D converter is two volts. This is due to the limited substrate size that is presently available.
If the converter must be sensitive to low input voltages, the electrodes must be very long. Because of the length, the capacitances of these electrodes limit the electrical frequency response of the interferometers. Consequently, the maximum speed of the analog-to-digital converter is reduced.